A grain-oriented electrical steel sheet is mainly utilized as an iron core of a transformer and is required to exhibit superior magnetization characteristics, in particular low iron loss.
In this regard, it is important to highly accord secondary recrystallized grains of a steel sheet with (110)[001] orientation, i.e., the “Goss orientation,” and reduce impurities in a product steel sheet. Furthermore, since there are limits on controlling crystal grain orientations and reducing impurities, a technique has been developed to introduce non-uniformity into a surface of a steel sheet by physical means to subdivide the width of a magnetic domain to reduce iron loss, i.e., a magnetic domain refining technique.
For example, JP S57-2252 B2 proposes a technique of irradiating a steel sheet as a finished product with a laser to introduce high-dislocation density regions into a surface layer of the steel sheet, thereby narrowing magnetic domain widths and reducing iron loss of the steel sheet. Furthermore, JP H6-072266 B2 proposes a technique of controlling the magnetic domain width by electron beam irradiation.
Thermal strain application-based magnetic domain refinement techniques such as laser beam irradiation and electron beam irradiation have the problem that insulating coating on the steel sheet is damaged by sudden and local thermal application, causing the insulation properties such as interlaminar resistance and withstand voltage, as well as corrosion resistance, to worsen. Therefore, after laser beam irradiation or electron beam irradiation, re-forming is performed on the steel sheet by applying an insulating coating again to the steel sheet and baking the insulating coating in a temperature range at which thermal strain is not eliminated. Re-forming, however, leads to problems such as increased costs due to an additional process, deterioration of magnetic properties due to a worse stacking factor, and the like.
A problem also occurs in that if the damage to the coating is severe, the insulation properties and corrosion resistance cannot be regained even by re-forming, and re-forming simply thickens the coating amount. Thickening the coating amount by re-forming not only worsens the stacking factor, but also damages the adhesion property and appearance of the steel sheet, thus significantly reducing the value of the product.
Against this background, techniques of applying strain while suppressing damage to the insulating coating have been proposed, for example, in JP S62-49322 B2, JP H5-32881 B2, JP 3361709 B2 and JP 4091749 B2. Specifically, to suppress damage to the coating, the methods disclosed in JP '252, JP '266, JP '322, JP '881 and JP '709 adopt approaches such as blurring the focus of the beam or suppressing the beam power to reduce the actual amount of thermal strain applied to the steel sheet. Even if the insulation properties of the steel sheet are maintained, however, the amount of iron loss reduction ends up decreasing. JP '749 discloses a method of reducing the iron loss while maintaining insulation properties by irradiating both sides of a steel sheet with a laser, yet that method is not advantageous in terms of cost, since irradiating both sides of the steel sheet increases the number of treatment steps.
It could therefore be helpful to provide a grain-oriented electrical steel sheet, on which magnetic domain refining treatment by strain application has been performed, having an insulating coating with excellent insulation properties and corrosion resistance.